Winding wheel for high-temperature baking of stacked capacitors

ABSTRACT

The invention pertains to a winding wheel for stacked capacitors, in the shape of a wheel body on which a rim is fixed in a detachable way, said rim having its circumference cut along a certain length and being clamped in a hoop made of a material with an expansion coefficient of zero. The cut length is calculated so that, when the rim and the hoop which clamps it are disconnected from the wheel body and carried to a determined temperature, the ends of the rim defined by the gap do not touch each other.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention pertains to the manufacture of capacitors of thetype that are stacked and obtained by winding on a large-diameter wheel.

2. Description of the Prior Art

This winding method is now widely used. Metallized dielectric films aregenerally used. The winding is done so as to make a lateral offsetbetween two successive foils of the stack so that electrical contactscan subsequently be deposited on the sides of the stack. Once the stackis made, it is subjected to heat treatment intended to stabilize itselectrical properties, especially the dielectric loss factor and thecapacitance value. This heat treatment also gets rid of air inclusionsbetween the foils of the stack. The treatment comprises an operation topress the stack directly on the winding wheel at fairly hightemperatures (about 240° C.). The winding wheel is generally made ofaluminum and its diameter is generally about 800 millimeters. The heattreatment causes the wheel to expand and consequently causes a majorelongation of the mother capacitors during baking. Between 20° C. and240° C., an aluminum wheel with a diameter of 800 mm. undergoes anelongation of 4 mm. in its diameter, corresponding to more than 12 mm.in its circumference.

The elongation that results from the mother capacitors counteracts thebeneficial effects of the baking process itself. To remove thisdisadvantage, it is possible to use wheels made of materials with a zeroexpansion coefficient, for example, invar. These materials are,unfortunately, very expensive and are not used in this specific casebecause they excessively increase the cost price of the capacitors.

To remove these disadvantages, the invention proposes the use of a wheelof a special design which makes it posssible to associate differentmaterials. The wheel according to the invention has several partsincluding, especially, a rim which is detachable from the wheel body andhas a radial gap so that it can expand freely under the effect of heat.This rim, which is made of a material generally used in this type oftechnology (aluminium for example), is clamped by a hoop made of amaterial with an expansion coefficient which is zero or practicallyzero. The hoop therefore sets a constant external diameter for the wheeland enables the free expansion of the rim.

3. Summary of the Invention

An object of the invention is therefore a winding wheel for stackedcapacitors, comprising a wheel body on which a rim is fixed detachably,the circumference of the said rim being cut along a certain length, therim being clamped within a hoop made of a material with an expansioncoefficient of zero, the said cut length being calculated so that, whenthe rim and the hoop which clamps it are disconnected from the wheelbody and carried to a determined temperature, the ends of the rimdefined by the cut do not touch one another.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood and its other advantages willappear from the following description, given as a non-exhaustive exampleand made with reference to the appended drawing, of which:

FIG. 1 shows the rim fitted with its hoop;

FIG. 2 is a detailed view of the FIG. 1;

FIG. 3 shows the winding wheel according to the invention;

FIG. 4 shows the stage for the heat processing of the mother capacitors.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, the rim 1 is clamped in the hoop 2. The rim is a relativelythick circle so that it can maintain a certain degree of rigidity whenit is suspended at one of its points. The material used to make it canbe aluminum, which is frequently used to make winding wheels. The rim isinterrupted along a certain length l. The diameter of the hoop 2 is suchthat, at winding temperature, the hoop clamps the rim 1. The gap in thecircle of the rim makes it is easier to position the hoop around therim. The rim can be fitted with grasping means so that the two ends ofthe cut circle can be brought closer together. If necessary, the hoopmay take the shape of a cut circle so that it can be fitted more easilyto the external diameter of the rim, or it may take the shape of a stripwhich is looped back on itself. The joint between the two ends of thehoop 2 must be clean so that the winding surface formed by the peripheryof the hoop is as smooth as possible.

FIG. 2 shows a possible embodiment of the closing of the hoop when it iscut. This figure corresponds to the part marked 3 in FIG. 1. Before thehoop is positioned around the rim, its two precisely machined ends arejoined, and a plate 5 with a curve substantially equal to that of thehoop is welded to the inner part of the hoop so as to fasten thempermanently. A groove 4 is made in the rim to house the plate 5 when thehoop is positioned on the rim. The plate 5 is preferably made of thesame material as the hoop.

The hoop should preferably be fixed to the rim at a point of itsperiphery. When the rim is heated, this type of fixing will contributegreatly to making the subsequent expansion cause its two ends to cometogether as shown by the two arrows in FIG. 1, rather than because ofany deformation of the hoop caused by the mechanical pressure exerted bythe rim. The sliding of the rim in the hoop on either side of theirfastening point will be all the easier if this fastening point isdiametrically opposite the gap made in the rim, as can be seen in FIG.1.

In the practical example shown in FIG. 2, two screws 6 and 7, placed oneither side of the gap in the hoop, have been chosen as fastening meansbetween the rim 1 and the hoop 2. These screws go through the plate 5and are fixed into the rim 1. Preferably, countersunk-head screws shouldbe chosen so that the external surface of the hoop stays even.

The hoop is made using a material with an expansion coefficient of zeroor practically zero, for example, invar. In the practical example of arim made of aluminium with a diameter of 0.8 m., a hoop made of invar,with a thickness of 2 mm., can be used if the hoop is fixed to a pointof the rim opposite to the gap of the said rim. The two parts of the rimcan then expand freely, and the mechanical resistance of the hoop issufficient to keep the wheel at a practically constant externaldiameter. The above rim, fitted with its hoop and carried to atemperature of 240° C., undergoes an expansion of 4/100 mm. in itsexternal diameter, thus causing an elongation of 12/100 mm. in thelength of the mother capacitors. This elongation is negligible withrespect to the length of the mother capacitors (about 2.5 m.).

The operation for winding the mother capacitors is performed on thewinding wheel shown in FIG. 3. The wheel has a wheel body 10 on whichthe rim 1, fitted with the hoop 2, is mounted. The wheel body has theconventional shape of a hub 11, with spokes 12 and a rim 13. The rim 1can be fixed to the wheel body 10 by any means known to the specialist.The fixing method chosen will preferably be one that enables rapiddismantling.

It has been mentioned above that the circumference of an aluminium rim,with a diameter of 800 mm., should be capable of being elongated byabout 12 mm. This elongation should take place in the cut portion of therim with a length l. The length l will therefore be at least equal to 12mm.

When the winding is done the rim, with its hoop and mother capacitorswhich are wound around it, is released from the wheel body and placed inan oven for the heat treatment of the mother capacitors. It is observedthat, to carry out the heat treatment, it is enough to suspend the rimin the oven. The wheel body remains on the winding machine and canreceive another rim. Another advantage of the invention is that there isno heat absorbed by the wheel body during heat treatment.

During the heat treatment process, the mother capacitors are usuallycovered with a holding and compacting hoop. It must be noted that, withthe winding wheel according to the invention, the use of a hooping madeof a material with a zero expansion coefficient is not obligatory.Advantageously, it is possible to use a hooping made of any metallicmaterial mounted with a calibrated compensating spring. In this case,the spring absorbs the expansions of the hoop with a constant force.This is shown in FIG. 4. The mother capacitors which have the genera;reference 20 are wound on the hoop 2 of the rim 1. A holding hoop 21clamps all the mother capacitors according to a technique which iswell-known to the specialist. To compensate for its expansion due to theheat treatment, the hoop 21 is fitted with a clamping device 22 providedwith a calibrated spring 23. Before the heat treatment, the hoop 21 isclamped to the mother capacitors by means of the screw 24 which goesfreely through the bore of the element 26 joined to one of the arms ofthe hoop 21, and which is screwed into the element 25 joined to theother arm. During the heat treatment, the expansion of the hoop 21causes the pressure that it exerts on the mother capacitors to berelaxed. The calibrated spring 23 placed between the element 26 and thehead of the screw 24 then fulfils its function and clamps the hoop 21against the mother capacitors.

The winding wheel of the invention can be used for the high-temperaturebaking of all dielectrics used for the manufacture of capacitors, inparticular so-called "japanned" dielectrics and, especially,thermostable dielectrics such as polyhydantoins, polyphenyquinoxalinesand polyimides.

The cost price of the winding supports is still low, with respect toboth the material used (chiefly aluminium) and the machining process.The results obtained are as good as they would be if the winding wheelwere made entirely of a material such as invar.

What is claimed is:
 1. A winding wheel for stacked capacitors,comprising:a hoop formed of a material having a substantially zerothermal coefficient of expansion; and an annular rim clamped within saidhoop so as to control an increase in the diameter of said rim, said hoopbeing fixed to said rim along a minor portion of the circumference ofsaid rim, the circumference of said rim having a discontinuity of acertain circumferential length, said length being sufficient that endsof said rim on opposite sides of said discontinuity do not touch oneanother at a temperature sufficient to heat treat capacitors wound onsaid winding wheel.
 2. A winding wheel according to the claim 1 whereinthe rim is made of aluminium.
 3. A winding wheel according to the claim1 wherein the hoop is made of invar.
 4. The winding wheel according toclaim 1, wherein said minor portion of the circumference of the rimsubstantially comprises two points, one of said points being at each ofsaid opposite ends.
 5. A winding wheel according to the claim 4 whereinthe hoop is fixed to the rim by means of a screws.
 6. A winding wheelaccording to claim 1 wherein the hoop is formed from a strip having twoends fixed to each other.
 7. A winding wheel according to the claim 6wherein the two ends of the said strip are fixed to each other bywelding.
 8. The winding wheel according to claim 7 wherein said minorportion of the circumference of the rim substantially comprises a point.9. The winding wheel according to claim 8 wherein said point isdiametrically opposite said discontinuity.
 10. The winding wheelaccording to claim 6 including plate means for fixing the two ends ofthe strip to one another.
 11. A winding wheel according to claim 10wherein said plate means comprises a plate made of the same material asthe hoop.
 12. A winding wheel according to the claim 10 wherein the saidrim comprises a groove designed to house the said plate means.